Stents are used by physicians to treat numerous conditions using minimally invasive procedures. Stents may be characterized as either balloon-expandable or self-expanding. Balloon-expandable stents are made from a ductile material that plastically deforms as the stent is compressed and expanded. Thus, when a balloon-expandable stent is delivered into a body passageway, an outward force must be applied to the stent to plastically expand it against the tissues of the passageway. Typically, an inflatable balloon is used to apply the outward expansion force to the stent. By contrast, self-expanding stents are made from an elastic material. A self-expanding stent is typically designed with an expanded diameter that is slightly greater in size than the body passageway that the stent will be implanted within. In the expanded diameter, the elastic material of the stent is unstressed, or relaxed.
In order to deliver a self-expanding stent, the stent must be compressed from its expanded diameter to a smaller delivery diameter. While compressed into the delivery diameter, the stent is loaded into a restraining sheath that prevents the stent from expanding back to its expanded diameter. When a physician is ready to deliver the self-expanding stent into a patient's passageway, the physician introduces the restraining sheath and the compressed stent into the patient and locates the stent at the desired treatment site. The stent is then pushed axially out of the distal end opening of the restraining sheath within the patient's passageway. This releases the stent from the restraining sheath, and as a result, the stent elastically expands back toward its expanded diameter until it contacts the tissues of the passageway.
Delivery systems for self-expanding stents suffer from a number of problems that can interfere with reliable delivery of self-expanding stents. Because the stent is elastically compressed inside of the restraining sheath as it is pushed out of the sheath during delivery, friction occurs between the outer surface of the stent and the inner surface of the restraining sheath. Thus, sufficient axial force must be applied to the stent and the restraining sheath in order to overcome this frictional force in order to release the stent from the sheath. High frictional forces between the stent and the restraining sheath can lead to numerous undesirable consequences. In extreme situations, the physician may not be able to apply sufficient force to the stent and sheath to release the stent from the sheath. However, even where the physician is able to overcome the frictional forces to deliver the stent, the frictional force between the stent and the sheath may interfere with accurate positioning of the stent.
High frictional forces between a self-expanding stent and restraining sheath during delivery affect a number of components in the delivery system. In order to push the stent axially out of the restraining sheath, an inner catheter is typically provided that abuts the proximal end of the stent or otherwise contacts a portion of the stent. During release of the stent, the position of the inner catheter is typically retained in place and the restraining sheath is typically pulled proximally relative to the inner catheter. The inner catheter is designed to prevent the stent from moving proximally with the restraining sheath as it is pulled proximally. As a result, the stent remains axially in place while the restraining sheath slides proximally over and away from the stent.
During delivery, the restraining sheath is in tension due to the pulling force at the proximal end and the frictional force with the stent at the distal end. In opposition, the inner catheter is in compression due to the axial restraining force at the proximal end and the stent pushing back against the inner catheter at the distal end due to the friction. As a result, the restraining sheath can stretch during delivery, and if sufficient force is applied, the sheath may partially or completely tear or otherwise fail. The inner catheter may also compress in length, which causes the stent to move proximally as the inner catheter changes in length, at least until some point when the frictional force drops and the inner catheter springs back to its original length. The inner catheter may also buckle within the restraining sheath, which contributes to the change in length of the inner catheter. The inner catheter typically has a guidewire lumen extending axially therethrough, and the inner catheter may also plastically clamp down on the guidewire, which would then require the delivery system and the guidewire to be withdrawn together from the patient. The forces on the restraining sheath and inner catheter during delivery typically require the device manufacturer to design the components from relatively stiff materials to withstand the expected forces. However, this can lead to larger diameter delivery systems and can lead to kinking of the restraining sheath and inner catheter.
Accordingly, the inventors believe it would be desirable to minimize frictional forces between a self-expanding stent and the restraining sheath during delivery of the stent.